Polarization conversion system

ABSTRACT

A reflector, light source, phase retarder, and linear polarizer configured to increase a single-polarization output beam by converting wrong-polarization light into correct-polarization light. The linear polarizer reflects the wrong-polarization light and transmits the correct-polarization light. The phase retarder converts the wrong-polarization light into correct-polarization light, which is then transmitted by the linear polarizer.

BACKGROUND OF THE INVENTION

[0001] 1. Technical Field of the Invention

[0002] The present invention relates generally to light projectors, andmore specifically to a projector light source with an improvedsingle-polarization output light beam.

[0003] 2. Background Art

[0004] In many applications, it is advantageous to employ a light sourceproviding a light beam of a single polarization. For example, when usedwith single-polarization light, spatial light modulators (SLMs) offerimproved visual quality of their modulated light output. It has beencommon practice to obtain the single polarization by discarding light ofthe other polarization. This is undesirable for many reasons, such asreduced output intensity, increased power consumption, increased heatgeneration, and so forth.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] The invention will be understood more fully from the detaileddescription given below and from the accompanying drawings ofembodiments of the invention which, however, should not be taken tolimit the invention to the specific embodiments described, but are forexplanation and understanding only.

[0006] FIGS. 1-5 show various embodiments of a polarization conversionsystem according to this invention.

[0007]FIG. 6 shows one embodiment of a method of operation of apolarization conversion system according to this invention.

DETAILED DESCRIPTION

[0008]FIG. 1 illustrates one embodiment of a polarization conversionsystem 10, including a light source 12 such as an arc, a plasma lamp, orother suitable means. In many applications, it will be desirable thatthe light source generate a light beam having a small etendue. Thesystem further includes a reflector 14 shaped and positioned relative tothe light source so to reflect the light beam into a substantiallycollaminated beam. The system further includes a quarter-wave phaseretarder 16 (“λ/4”) which rotates the polarization of the light passingthrough it by 45 degrees. The system also includes a linear polarizer 18which transmits light of a first polarization and reflects light ofanother polarization. One embodiment of a suitable linear polarizer isan optical substrate with a micro-structured wire grid type polarizersurface.

[0009] Operation of the system will be explained with regard to lightthat is reflected by the reflector. The reader will understand that thesame principles apply to light that travels directly from the lightsource to the polarizer without first being reflected by the reflector.The light emerges from the light source with a random polarization(L_(random)), is reflected by the reflector, passes through thequarter-wave phase retarder, and encounters the linear polarizer. Someof the light (L_(parallel0)) happens to be of the correct polarization,and is transmitted through the linear polarizer to emerge as a firstcomponent of the output beam.

[0010] Some of the light (L_(orthogonal)) is of the wrong polarization,and is reflected by the linear polarizer. It then passes back throughthe quarter-wave phase retarder, which rotates its polarization by 45degrees, is reflected by the reflector, and passes again through thequarter-wave phase retarder, where its polarization is rotated by anadditional 45 degrees. Now, when it encounters the linear polarizer, thelight is of the correct polarization (having made two passes through thequarter-wave phase retarder, for a total of a half phase of polarizationshift), and it emerges as a second component of the output beam(L_(parallel1)).

[0011] Unless ideal components can be utilized, there will be losses asthe light beams traverse the system. The better the components, thelower the losses, and the greater contribution the second component(L_(parallell)) makes to the overall brightness of the output beam.

[0012] One suitable linear polarizer is the ProFlux polarizer availablefrom Moxtek, Inc, 452 W 1260 N, Orem, Utah 84057. Some details of thelinear polarizer are shown in U.S. Pat. Nos. 6,108,131, 6,122,103, and6,288,840. Additional information regarding such polarizers can be foundat http://www.moxtek.com and http://www.profluxpolarizer.com.

[0013]FIG. 2 shows a second embodiment 20 of a system utilizing thisinvention. It contains a light source 12, reflector 14, and linearpolarizer 18 as in the first embodiment. However, the phase retarder 26is layered on the reflector rather than being configured with thepolarizer. The phase retarder coating causes a quarter-wave retardationwhen passed through twice (passing into, then reflected back outthrough). In some embodiments, this coating may retard differently inthe two directions, and may retard differently depending upon the angleof incidence.

[0014]FIG. 3 illustrates a third embodiment of a polarization conversionsystem 30. It includes a light source 12, as well as a quarter-wavephase retarder 16 and a linear polarizer 18. The retarder and polarizercan, in this embodiment, be made approximately half the size they wouldbe in the other embodiments, because the reflector 32 is approximatelyone half the size of the reflectors in the embodiments of FIGS. 1 and 2.The system further includes a reflector 36. The reflector 36 issubstantially hemispherical, with a reflective concave surface.

[0015] Correct polarization light from the light source will betransmitted by the linear polarizer as a first component (L_(parallel0))of the output beam. Light of the wrong polarization will be reflected bythe polarizer, off the reflector 32, through the light source into theinterior chamber of the reflector 36 combination. Eventually, it willbounce back through the light source, off the reflector 32, make anotherpass through the retarder, and be of the correct polarization when itarrives at the polarizer, from which it will emerge as a secondcomponent (L_(parallel1)) of the output beam. In some embodiments, thelight will be re-emitted as randomly polarized light, rather than merelypassing through as indicated above. The embodiment of FIG. 3 will, ofcourse, generate a narrower output beam than those of FIG. 1 or 2,assuming the same general scale of components.

[0016]FIG. 4 shows a fourth embodiment 40 of a system utilizing thisinvention. It includes a radio-frequency (RF) driven plasma light source42, such as that available from Fusion, Inc. of 7524 Standish Place,Rockville, Md., USA. The light source includes a quartz bulb 44 to whichis affixed a quarter-wave phase retarder 16 and within which is asubstantially spherical light cavity. In some embodiments, the quartzbulb may have a flat spot to facilitate bonding of the phase retarder.The light source also includes a specular high reflectivity ceramiccoating 46 or other suitable reflector, which is advantageouslydeposited so as to encapsulate the bulb and the phase retarder. Thereflector includes a substantially non-reflective opening 48 which may,in some embodiments, be located substantially opposite the phaseretarder. The angles from the perimeter of the phase retarder to theedges of the opening define a field of vision, denoted in FIG. 4 bydotted lines. Ideally, the linear polarizer 18 should encompass thisfield of view. In some embodiments, the linear polarizer could belocated within the opening 48.

[0017] Some of the light created by the RF-induced plasma (not shown)will escape through the opening and contact the linear polarizer. Aportion of that light that is of the correct polarization will betransmitted as a first component (L_(parallel0)) of the output beam. Thewrong-polarization portion will be reflected back into the plasma. Afterperhaps making some number of bounces off the reflective coating 46, thewrong polarization light may pass through the phase retarder, bereflected back through the phase retarder, pass through the opening 48,and be transmitted through the linear 3 polarizer as a second component(L_(parallel1)) of the output beam.

[0018]FIG. 5 shows a fifth embodiment 50 of a system similar to that ofFIG. 4. The light source 52 has a reflective coating (not shown) on itsinterior surface, and a first 54 and a second 56 non-reflective windowthrough which the light beam can emerge. It includes a reflector 14,quarter-wave phase retarder 16, and linear polarizer 18, which are sizedappropriately. Light emerges from the first window and contacts thelinear polarizer. If it happens to be of the correct polarization, it istransmitted as a first component of the output beam. Otherwise, it isreflected back through the first window, emerges from the second window,passes through the quarter-wave phase retarder, is reflected by thereflector, makes a second pass back through the quarter-wave phaseretarder, passes through the second window, and emerges from the firstwindow. This time when it encounters the linear polarizer, it is of thecorrect polarization, having made two passes through the quarter-wavephase retarder, and becomes a second component of the output beam.

[0019]FIG. 6 illustrates one exemplary embodiment 60 of a method ofoperation of a polarization conversion system utilizing this invention.Light is generated (61) with plural polarizations, typically randompolarization. It is conducted (62) to a polarizer. If (63) a componentof the light is of the correct polarization, it is transmitted (64) asone component of an output beam. If, however, a component of the lightis of the wrong polarization, it is reflected (65) for correction. Inthe process of correction, the phase of the light's polarization isaltered (66). In embodiments, the phase-altered light is then reflected(67) back toward the polarizer. In some embodiments, its phase may againbe altered (68), if that is necessary in order for it to be of the rightpolarization. In other embodiments, it may be possible to achieve thecorrect polarization in a single operation, such as by passing itthrough a half-wave phase retarder. If the half-wave phase retarder isonly operative in a single direction, the phase-altered light may bereflected directly back through the (in this direction inert) phaseretarder on its way to the polarizer. If not, it will be necessary tobypass the phase retarder such as by reflecting around it by a series ofmirrors. Ultimately, the corrected-polarization light is transmitted(64) as a second component of the output beam. In other embodiments, itmay be desirable to use a phase retarder which rotates the light somefraction of a phase other than one quarter or one half. In those cases,the wrong-phase light may bounce between the polarizer and the reflectorseveral times before its polarization finally becomes correct and itemerges from the polarizer.

[0020] Reference in the specification to “an embodiment,” “oneembodiment,” “some embodiments,” or “other embodiments” means that aparticular feature, structure, or characteristic described in connectionwith the embodiments is included in at least some embodiments, but notnecessarily all embodiments, of the invention. The various appearances“an embodiment,” “one embodiment,” or “some embodiments” are notnecessarily all referring to the same embodiments.

[0021] If the specification states a component, feature, structure, orcharacteristic “may”, “might”, or “could” be included, that particularcomponent, feature, structure, or characteristic is not required to beincluded. If the specification or claim refers to “a” or “an” element,that does not mean there is only one of the element. If thespecification or claims refer to “an additional” element, that does notpreclude there being more than one of the additional element.

[0022] Those skilled in the art having the benefit of this disclosurewill appreciate that many other variations from the foregoingdescription and drawings may be made within the scope of the presentinvention. Indeed, the invention is not limited to the details describedabove. Rather, it is the following claims including any amendmentsthereto that define the scope of the invention.

What is claimed is:
 1. An apparatus comprising: a reflector; a lightsource for producing light of a first polarization and light of a secondpolarization; a linear polarizer which substantially passes light of thefirst polarization and substantially reflects light of the secondpolarization; and a phase retarder disposed between the reflector andthe linear polarizer.
 2. The apparatus of claim 1 wherein the phaseretarder comprises a quarter-wave phase retarder.
 3. The apparatus ofclaim 1 wherein the light source is disposed between the reflector andthe phase retarder.
 4. The apparatus of claim 1 wherein the linearpolarizer comprises an optical substrate with a micro-structured wiregrid type polarizer surface.
 5. The apparatus of claim 4 wherein thelinear polarizer further comprises airflow gaps for cooling the linearpolarizer.
 6. The apparatus of claim 1 wherein the reflector comprises asubstantially parabolic mirror, and wherein the light source is disposedsubstantially at a focus of the parabolic mirror.
 7. The apparatus ofclaim 6 wherein the linear polarizer comprises a substantially planarstructure.
 8. The apparatus of claim 6 wherein the linear polarizer andthe phase retarder are suitably sized to collect light emitted from thereflector.
 9. The apparatus of claim 1 wherein the light source isdisposed between the phase retarder and the linear polarizer.
 10. Theapparatus of claim 9 wherein the phase retarder is mechanically coupledto the reflector and has substantially a same shape as the reflector.11. The apparatus of claim 10 wherein the phase retarder and thereflector are substantially parabolic.
 12. The apparatus of claim 9wherein the light source comprises a substantially spherical lightcavity having a reflective inner surface, and having a first window anda second window through the reflective inner surface, the first andsecond windows being disposed at substantially opposite sides of thelight cavity.
 13. The apparatus of claim 12 wherein the phase retarderand the reflector are disposed substantially in an optical path with thefirst window, and the linear polarizer is disposed substantially in anoptical path with the second window.
 14. The apparatus of claim 13wherein the phase retarder, the reflector, and the linear polarizer areof substantially planar shape.
 15. The apparatus of claim 9 wherein thelight source comprises: an RF-driven plasma light source having a bulb,and having a reflective coating surrounding the bulb and the phaseretarder.
 16. An apparatus comprising: a curved reflector having aconcave reflective surface; a quarter-wave phase retarder in an opticalpath with the curved reflective surface; and a linear polarizer in theoptical path.
 17. The apparatus of claim 16 further comprising: a lightsource in the optical path.
 18. The apparatus of claim 16 wherein thecurved reflector has a substantially parabolic shape.
 19. The apparatusof claim 16 wherein the curved reflector has a substantiallysemi-parabolic shape.
 20. The apparatus of claim 18 wherein thequarter-wave phase retarder and the linear polarizer are ofsubstantially planar shape.
 21. The apparatus of claim 20 furthercomprising: a light source in the optical path and having asubstantially spherical shape, a reflective interior, and two windows ofreduced reflectivity, one of the windows positioned to transmit light tothe quarter-wave phase retarder and the reflector, and the other of thewindows positioned to transmit light to the linear polarizer.
 22. Theapparatus of claim 18 wherein the quarter-wave phase retarder has thesubstantially parabolic shape and the linear polarizer has asubstantially planar shape.
 23. A system comprising: linear polarizermeans for transmitting light of a first polarization and for reflectinglight of a second polarization; phase retarder means for alteringpolarization of the light of the second polarization reflected from thelinear polarizer means; and reflector means for reflecting the alteredpolarization light from the phase retarder means back to the linearpolarizer means.
 24. The system of claim 23 wherein the reflector meansis disposed so as to reflect the altered polarization light back throughthe phase retarder means, and wherein the phase retarder means isfurther for re-altering polarization of the light from the reflector.25. The system of claim 24 wherein the phase retarder means is foraltering the polarization of the light from the linear polarizer by onequarter phase and the light from the reflector by one quarter phase, toimpart a one half phase alteration in polarization to light passingthrough it twice in the course of bouncing between the linear polarizerand the reflector.
 26. The system of claim 23 further comprising meansfor providing the light of the first and second polarizations.
 27. Thesystem of claim 26 wherein the means for providing the light is disposedbetween the phase retarder means and the reflector means.
 28. The systemof claim 26 wherein the means for providing the light is disposedbetween the phase retarder means and the linear polarizer means.
 29. Thesystem of claim 23 wherein the reflector means comprises a substantiallyparabolic reflector.
 30. The system of claim 23 wherein the reflectormeans comprises a substantially planar reflector.
 31. The system ofclaim 23 further comprising a substantially spherical light source witha reflective inner surface.
 32. A method comprising: transmitting firstcorrect-polarization light through a linear polarizer; reflectingwrong-polarization light from the linear polarizer; correctingpolarization of the wrong-polarization light to make it secondcorrect-polarization light; and transmitting the secondcorrect-polarization light through the linear polarizer.
 33. The methodof claim 32 further comprising: reflecting by a reflector light that hasbeen reflected from the linear polarizer.
 34. The method of claim 32wherein the correcting comprises: first altering polarization of thewrong-polarization light; reflecting the altered-wrong-polarizationlight; and second altering polarization of thealtered-wrong-polarization light to make it the secondcorrect-polarization light.
 35. The method of claim 34 wherein the firstand second altering each comprises: passing its respectivewrong-polarization light through a quarter-wave phase retarder.
 36. Themethod of claim 35 further comprising: generating the firstcorrect-polarization light and the wrong-polarization light.